Invar 36 is a material that inspires both admiration and frustration among machinists.
Admiration because it’s a metallurgical marvel: an iron-nickel alloy with a coefficient of thermal expansion so low it barely moves from room temperature to 200°C. It’s the material that makes precision optics, aerospace tooling, and semiconductor equipment possible.
Frustration because machining it is brutal. Invar 36 is gummy and soft like stainless steel’s difficult cousin. It work-hardens rapidly. It generates intense cutting heat. The tool wear is merciless. And if you don’t manage internal stress during machining, the part will warp—ironic for a material famous for not expanding.
Very few shops want to touch Invar 36. That’s precisely why we’ve made it a core capability at Baosheng Industry.
In this guide, we’ll share the specific strategies—stress relief, tooling, parameters, and cooling—that we use to machine Invar 36 parts to ±0.001″ and 32 Ra surface finish. If you’re an engineer designing with Invar, or a machinist struggling with it, this is for you.
Explore our CNC machining services for Invar and other exotic alloys →
What Makes Invar 36 So Difficult to Machine?
Invar 36 (also known as FeNi36, Pernifer 36, or UNS K93600) is a high-nickel alloy containing approximately 36% nickel and 64% iron. Its claim to fame is a coefficient of thermal expansion (CTE) of approximately 1.2 × 10⁻⁶ /°C—about one-tenth that of carbon steel.
But the metallurgical properties that give Invar its low expansion also make it a machining nightmare.
| Property | Value | Machining Implication |
|---|---|---|
| Nickel Content | 36% | High work-hardening rate; gummy cutting behavior |
| Thermal Conductivity | ~10–13 W/m·K (very low) | Heat concentrates at the cutting edge—200–300°C tool tip temps |
| Tensile Strength (annealed) | 450–550 MPa | High cutting forces required |
| Elongation | 30–40% | Ductile; forms long, stringy chips that tangle and recut |
| Work-Hardening Rate | High | Surface hardness can increase by 50–100% after a single pass |
In short, Invar 36 combines the worst of several worlds: the low thermal conductivity of titanium, the gumminess of nickel alloys, and the work-hardening tendency of austenitic stainless steel. It’s a trifecta that punishes inexperienced machinists.
The good news: Invar 36 is machinable with the right process. The key is understanding its behavior and respecting its limits—starting with stress relief.
Learn how manual DFM review catches Invar-specific issues that automated software misses →
1. Stress Relief: The Non-Negotiable First Step
If you take away one lesson from this guide, make it this: you cannot skip stress relief when machining Invar 36.
Invar 36 arrives from the mill with significant residual stress from the rolling or forging process. When you remove material, those stresses redistribute. The part warps. Dimensional tolerances go out the window.
Worse, the thermal and mechanical stresses introduced during machining can accumulate and cause post-machining dimensional drift—the part slowly changes shape over hours or days after it comes off the machine. For aerospace and optical applications, this is a catastrophe.
Our Invar 36 Stress Relief Protocol
We apply a thermal stress relief cycle at two critical points during the manufacturing process:
| Stage | Process | Temperature | Duration | Purpose |
|---|---|---|---|---|
| Pre-Machining | Full anneal in vacuum or inert atmosphere | 815–870°C (1500–1600°F) | 1 hour per 25mm of section thickness, then furnace cool to 315°C (600°F) | Removes mill residual stress; homogenizes material |
| Post-Roughing | Stress-relief anneal | 315–425°C (600–800°F) | 2–4 hours, slow cool | Relieves machining-induced stress before finishing |
Critical detail: The post-roughing stress relief is performed after rough machining leaves 0.015–0.030″ stock. This allows the part to warp freely, and the warped geometry is corrected in the finishing pass.
What happens if you skip stress relief:
- Rough machined flatness within 0.002″ can become 0.010″ out of flat after finishing due to stress redistribution.
- Tight-tolerance bores can go oval by 0.0005–0.0015″.
- Parts may drift for days after final inspection.
Cost of proper stress relief: The thermal cycles add 1–2 days to the overall process. The cost of skipping them? Scrapped parts, missed deadlines, and customer rejection.
See our full material capabilities including exotic alloys →
2. Tool Selection: Carbide Grade, Geometry, and Coatings
Invar 36 eats the wrong tools for breakfast. Choosing the right tooling is critical.
Carbide Grade
Use a sub-micron grain carbide with high cobalt content (10–12%) and high transverse rupture strength. The tool needs toughness to resist the abrasive wear and micro-chipping caused by Invar’s work-hardened surface.
| Carbide Grade | Recommendation | Notes |
|---|---|---|
| ISO K10–K20 (Cast Iron Grade) | ❌ Not recommended | Too brittle; prone to chipping |
| ISO P30–P40 (Steel Grade with Toughness) | ✅ Good | Tough, but still vulnerable to diffusion wear |
| ISO M30–M40 (Stainless/Nickel Grade) | ✅✅ Best | Optimized for nickel alloys; good wear resistance and toughness |
| Sub-micron 10% Co | ✅✅ Best for finishing | Fine grain size maintains sharp edge; resists built-up edge |
Coatings
Coatings for Invar 36 must handle high heat and resist the nickel-iron diffusion that causes crater wear.
| Coating | Effectiveness for Invar 36 | Notes |
|---|---|---|
| Uncoated Carbide | ❌ Poor | Bare carbide reacts with nickel at high temperatures; rapid wear |
| TiN (Titanium Nitride) | ⚠️ Moderate | Basic protection; wears quickly at high speeds |
| TiAlN (Titanium Aluminum Nitride) | ✅ Good | Oxide layer protects at high temperatures; good all-around |
| AlTiN (Aluminum Titanium Nitride) | ✅✅ Better | Higher aluminum content forms more stable oxide; excellent for dry/roughing |
| AlCrN (Aluminum Chromium Nitride) | ✅✅ Best | Superior hot hardness and oxidation resistance up to 1100°C |
Our shop standard for Invar 36: AlTiN-coated sub-micron carbide with a sharp, positive geometry for roughing and semi-finishing; AlCrN-coated for finishing passes where surface finish is critical.
Tool Geometry
| Parameter | Recommendation | Why |
|---|---|---|
| Rake Angle | Positive, 8°–12° | Reduces cutting force; prevents work-hardening from rubbing |
| Helix Angle | Variable helix (38°–42°) | Breaks up harmonics; reduces chatter in deep cuts |
| Flute Count | 4–5 for roughing (rigidity); 4–6 for finishing | More flutes = more rigid core, but ensure chip room |
| Corner Radius | 0.015–0.030″ (0.4–0.8 mm) | Avoid sharp corners—they chip immediately |
| Edge Prep | Light hone (0.0005–0.001″) | Protects cutting edge without excessive force |
Key rule: Never use a tool that has previously cut steel or stainless on Invar 36. Any edge wear from prior jobs will be amplified by Invar’s work-hardening and lead to catastrophic failure within minutes.
Read our DFM guide to ensure your Invar part design is machinable →
3. Cutting Parameters: Speeds, Feeds, and Depth of Cut
Invar 36 demands conservative cutting parameters compared to aluminum or carbon steel. The low thermal conductivity means heat must be managed through lower speeds and appropriate chip loads—not just more coolant.
Recommended Parameters for Invar 36 (FeNi36)
| Operation | Surface Speed (SFM) | Feed per Tooth (IPT) | DOC Radial (Rough) | DOC Axial (Rough) | DOC (Finish) |
|---|---|---|---|---|---|
| Side Milling (Rough) | 150–250 | 0.004–0.008 | 0.15–0.30 × D | 0.5–1.0 × D | — |
| Side Milling (Finish) | 200–300 | 0.002–0.004 | 0.010–0.030″ | Full depth | 0.005–0.010″ |
| Slot Milling | 120–180 | 0.003–0.006 | Full slot | 0.25–0.5 × D | — |
| Drilling (HSS-Co or Carbide) | 50–80 | 0.002–0.005 / rev | — | Peck every 0.5–1.0×D | — |
| Tapping | 20–30 | Per tap spec | — | 60–65% thread | — |
| Reaming | 30–50 | 0.002–0.004 / rev | 0.005–0.010″ stock | — | — |
Critical notes:
- Do not exceed 250 SFM for roughing with AlTiN-coated carbide. At higher speeds, the tool tip temperature can exceed 800°C, causing rapid diffusion wear.
- Maintain a minimum chip thickness of 0.002 IPT. Below this, the tool rubs instead of cuts, work-hardening the surface and causing early tool failure.
- Peck drilling is mandatory. Chips pack in deep holes and cause tool breakage. Peck every 0.5–1.0× diameter, with full retraction to clear chips.
How We Dial In Stable Parameters
For every new Invar 36 job, our lead machinists perform a quick stability test:
- Start at the low end of the speed range (150 SFM).
- Perform a short test cut and listen for chatter, monitor chip formation.
- If chips are short and curled (ideal), increase speed in 10% increments until chip color or sound changes.
- If chips are stringy or tool shows built-up edge, reduce speed or increase feed slightly.
- Once stable parameters are found, lock them for the production run.
Learn how our factory-direct approach avoids generic Invar parameters that ruin parts →
4. Coolant Strategy: Flood, MQL, or Dry?
Invar’s low thermal conductivity means heat pools at the cutting edge. Cooling is essential—but the method matters.
| Coolant Method | Effectiveness for Invar 36 | Notes |
|---|---|---|
| Dry Machining | ❌ Avoid | Heat builds rapidly; tool life drops to minutes |
| Flood Coolant | ✅✅ Best for roughing | High volume (≥ 10 gal/min) with 8–10% concentration of a high-quality semi-synthetic or synthetic coolant. Focused nozzles directly at the cutting zone are essential |
| MQL (Minimum Quantity Lubrication) | ⚠️ Marginal | Does not remove enough heat for Invar. Acceptable only for very light finishing cuts in thin sections |
| High-Pressure Through-Spindle | ✅✅ Best for drilling/deep cavities | 1000+ psi coolant flushes chips from deep holes and prevents re-cutting |
Our shop standard:
- Roughing: Heavy flood coolant (≥ 12 gal/min), twin nozzles aimed at the tool entry and exit points.
- Finishing: Flood coolant with slightly lower volume, but directed precisely. Adding a mist or MQL overlay for lubrication can improve surface finish on finishing passes.
- Drilling: Through-spindle coolant at 70+ bar (1000+ psi) with carbide drills. If through-spindle isn’t available, peck drilling with external flood is mandatory.
Coolant maintenance for Invar: Invar chips are fine and abrasive. They can clog coolant nozzles and filters. We increase coolant filtration frequency to every 50 operating hours when running Invar jobs.
Explore our aerospace machining capabilities where Invar is frequently used →
5. Chip Control and Surface Finish Strategies
Invar 36 produces long, stringy chips that wrap around the tool, scratch the part, and cause re-cutting. Controlling chips is as important as controlling cutting parameters.
Chip Control Tactics
| Issue | Solution |
|---|---|
| Long, continuous chips | Increase feed rate slightly (0.001–0.002 IPT) to break chips; use chip breaker geometry inserts |
| Chip nesting in cavity | Use through-spindle coolant; program periodic chip-clearing retracts |
| Chip re-cutting and surface scratching | Flush continuously; consider air blast in addition to coolant |
| Built-up edge on tool | Increase speed slightly; switch to a sharper, more polished coating |
Achieving a Good Surface Finish on Invar 36
A 32 Ra surface finish is achievable with careful finishing. A 16 Ra finish requires extra attention.
| Strategy | Surface Finish Improvement |
|---|---|
| Light finishing DOC (0.005–0.010″) | Prevents work-hardening; yields more consistent finish |
| High-positive, sharp tool (AlCrN coated) | Reduces friction and smearing |
| Stable setup, short tool overhang | Prevents micro-chatter that shows on the surface |
| Climb milling on all finish passes | Cleaner shearing action |
| Fresh, dedicated finishing tool | Eliminates tool wear as a variable |
| Post-process stress relief | Stabilizes surface integrity for critical applications |
A note on dimensional stability after machining: Even with proper in-process stress relief, Invar 36 parts can exhibit a small amount of dimensional drift over the first 24–48 hours after final machining. For critical ±0.0005″ tolerances, we let parts rest at room temperature for 24 hours before final inspection—or perform a low-temperature stabilization cycle at 200°C for 2 hours, then cool slowly.
Read about our tight-tolerance CNC machining for exotic alloys →
6. Real-World Case Study: Invar 36 Optical Bench
An aerospace optics manufacturer approached us with a challenging Invar 36 component:
The Part: An optical bench (base plate) for a satellite instrument.
- Dimensions: 400mm × 300mm × 25mm (15.7″ × 11.8″ × 1.0″)
- Key features: Flatness 0.0005″ over the entire surface; 48 precision dowel holes with true position tolerance 0.001″; 32 Ra surface finish on mounting faces.
- Material: Invar 36 plate, annealed condition.
The Challenges Identified:
- The large, thin aspect ratio would make stress-related warping inevitable without proper protocol.
- Holding 0.0005″ flatness over a 400mm length in Invar 36 is pushing the limit.
- 48 precision holes mean many cutting cycles, each introducing local stress.
Our Approach:
- Pre-machining anneal at 850°C for 2 hours, furnace cooled.
- Rough machining with 0.060″ stock on all surfaces, using AlTiN-coated carbide, 200 SFM, flood coolant.
- Post-rough stress relief at 400°C for 3 hours, slow cool to ambient.
- Semi-finish pass leaving 0.015″ stock. Measured flatness at this stage: 0.003″.
- Finish machining with fresh AlCrN-coated carbide tools, 250 SFM, 0.003 IPT, 0.008″ DOC. Climb milling throughout. Used a vacuum fixture with full-surface support to maintain flatness during the finishing cut.
- Final inspection after 24-hour room-temperature stabilization. Verified true position of all 48 holes with CMM.
Results:
- Flatness: 0.0004″ (exceeded 0.0005″ requirement).
- True position dowel holes: all within 0.0008″.
- Surface finish: 25–30 Ra on mounting faces.
- Zero scrap. Delivered on time.
Client impact: The part successfully passed satellite integration testing and is scheduled for flight. This is the kind of result that comes only from a shop that understands Invar’s personality—not from automated quoting platforms or general-purpose shops.
Learn more about our precision CNC machining for aerospace →
Invar 36 Machining Quick-Reference Card
| Parameter | Roughing | Finishing |
|---|---|---|
| Tool Material | Sub-micron carbide, AlTiN coated | Sub-micron carbide, AlCrN coated |
| Rake Angle | 8°–10° positive | 10°–12° positive |
| Surface Speed (SFM) | 150–250 | 200–300 |
| Feed per Tooth (IPT) | 0.004–0.008 | 0.002–0.004 |
| Radial DOC | 0.15–0.30 × D | 0.010–0.030″ |
| Axial DOC | 0.5–1.0 × D | 0.005–0.015″ |
| Coolant | Heavy flood (≥12 gal/min) | Flood + optional MQL overlay |
| Stress Relief | Post-rough: 315–425°C, 2–4 hrs | Post-finish: optional stabilization |
| Achievable Tolerance | — | ±0.0005″ (with stabilization) |
| Achievable Surface Finish | — | 16–32 Ra |
Need Help with Your Invar 36 Parts?
At Baosheng Industry, we don’t shy away from difficult materials. Invar 36 is one of our specialties, and we’ve built our process—stress relief, tooling, parameters, and inspection—around its unique behavior.
Unlike automated platforms that apply generic “iron-nickel alloy” parameters and hope for the best, we approach every Invar 36 part with a material-specific strategy refined over hundreds of jobs. We offer a free manual DFM analysis within 24 hours that addresses stress relief planning, fixturing for dimensional stability, and toolpath optimization for Invar.
Upload your CAD file today and let us show you the difference a factory that knows Invar can make.
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Frequently Asked Questions (FAQ)
Q1: Why is Invar 36 so difficult to machine?
Answer: Invar 36 combines high nickel content (36%), low thermal conductivity, high ductility, and rapid work-hardening. It generates intense cutting heat, forms long stringy chips, and wears tools quickly. Proper stress relief, tool selection, and conservative parameters are essential.
Q2: What stress relief process do you use for Invar 36?
Answer: We apply a two-stage stress relief: a full anneal at 815–870°C before any machining to remove mill residual stress, and a post-roughing stress relief at 315–425°C for 2–4 hours before finishing. This prevents warping and dimensional drift.
Q3: What cutting tools are best for Invar 36?
Answer: Sub-micron carbide with tough cobalt binder (10–12%), coated with AlTiN (roughing) or AlCrN (finishing). Positive rake angles (8°–12°) and variable helix geometry improve chip control and reduce work-hardening.
Q4: Can Invar 36 be machined on a standard CNC mill?
Answer: Yes, with the right tooling, parameters, and stress relief protocol. Standard 3-axis and 5-axis CNC mills work well. Rigidity is critical—use hydraulic or shrink-fit holders, minimize stick-out, and ensure the machine has adequate coolant delivery.
Q5: What surface finish can be achieved on Invar 36?
Answer: A 32 Ra surface finish is routinely achievable with optimized finishing parameters and fresh tooling. 16 Ra is possible with additional care, including sharp AlCrN-coated tools, light finishing cuts, and post-process stabilization.
Q6: Does Invar 36 warp after machining, even with stress relief?
Answer: With proper stress relief protocol, warping is minimized. However, Invar can exhibit minor dimensional drift over 24–48 hours after final machining. For critical tolerances, we include a post-machining stabilization period or a low-temperature thermal cycle at 200°C.
Q7: How does Baosheng’s approach differ from automated quoting platforms for Invar 36?
Answer: Automated platforms typically apply generic “iron-nickel alloy” parameters. They do not plan stress relief, and they often reject or heavily surcharge Invar jobs. Baosheng applies a material-specific process including stress relief planning, tailored tooling, and direct engineer communication.
Q8: What industries use Invar 36 machined parts?
Answer: Invar 36 is used in aerospace (satellite optical benches, composite tooling), semiconductor manufacturing equipment, precision measurement instruments, and cryogenic components—anywhere dimensional stability over a wide temperature range is critical.
